1. Field of the Invention
This invention relates to a cold cathode electron source element and a method for manufacturing the same.
2. Background Art
Field emission type electron sources can be manufactured to a micron size by virtue of semiconductor micro-processing technology and are easy to integrate and process batchwise. They are expected to find application in GHz band amplifiers and high-power/high-speed switching elements, to which thermionic emission type electron sources could not be applied, as well as electron sources for high definition flat panel displays. Active research and development efforts have been made thereon over the world.
Prior art examples of the field emission type electron source are described below. Japanese Patent Application Kokai (JP-A) No. 274047/1988 proposes a thin film field emission type electron source which includes, as shown in FIG. 35, a cold cathode 52 and an opposing gate electrode 53 deposited on an insulator substrate 51 with a spacing of 0.3 to 2 .mu.m wherein a voltage is applied across the cold cathode 52 and the gate electrode 53 in vacuum to induce electron emission. This cold cathode 52 is formed using a FIB (focused ion beam) technique, especially with the end of convergent fingers being sharply pointed. The FIB technique used herein, however, makes it difficult to manufacture an element with a large surface area and increases the manufacturing cost.
When a large surface area and manufacturing cost are taken into account, on the other hand, patterning by photolithography is deemed appropriate. However, the current photolithography is limited to a patterning diameter of the order of 0.5 .mu.m since the diameter of an electron beam spot is the minimum patterning diameter. As a consequence, in order to form the cold cathode 52 with sharply pointed fingers, various additional steps must be taken. As the number of steps increases, the possibility of damaging the element, especially its cold cathode finger tips increases, which causes a lowering of the manufacturing yield of elements. Most of the cold cathode sharpening steps are complex and difficult to control the shape.
JP-A 49129/1991 proposes a thin film field emission type electron source in which as shown in FIG. 36, a cold cathode 63 and a gate electrode 64 are formed parallel on the surface of an insulating layer 62 on an insulator substrate 61 by a cleavage and fracture process using ultrasonic wave. The thin film field emission type electron source shown in FIG. 36), however, has the problems that because of the concomitant ultrasonic fracture, formation of the cold cathode 63 to a uniform shape is technically difficult and the thin film from which the cold cathode 63 is formed receives substantial damages.
JP-A 252025/1991 proposes a thin film field emission type electron source in which as shown in FIGS. 37 and 38, a cold cathode 73 having a plurality of convergent fingers are formed on an insulating layer 72 on an insulator substrate, 71 by a photo-etching technique and thereafter, the convergent fingers at their tip are sharply pointed utilizing an isotropic etching technique. It is noted that 74 in FIG. 37 is a gate electrode 74 opposed to the cold cathode 73. In this electron source, however, it is difficult to control the shape of the cold cathode 73 by a choice of etching conditions. The process is not applicable where no undercutting takes place due to formation of a side wall protecting film or some other reasons.
Also JP-A 220337/1990 discloses to coat the cold cathode 73 on its surface with a transition metal carbide, metal oxide or rare earth oxide that is a low work function material which is chemically stable and likely to emit electrons into vacuum. However, it is difficult to limit such coating to the cold cathode 73 and the like.
As mentioned above, in the case of prior art field emission electron sources, the shape of a cold cathode including pointing of cold cathode finger tips could not be properly defined and it was impossible to use a low work function, chemically stable material as the cold cathode because of difficulty of micro-processing. These undesirably inhibited manufacture of a stable field emission electron source having satisfactory properties.
U.S. Pat. No. 5,019,003 discloses a field emitting device having a plurality of preformed emitter (or cold cathode) particles distributed on a support. In this device, as shown in FIG. 39, a plurality of conductive objects 201 are distributed on a support substrate 100, the conductive objects 201 being coupled to the substrate 100 by a bonding agent 101. The conductive objects 201 may be of molybdenum, titanium carbide or the like, preferably have geometrically sharp edges, and function as emitters. It is described that instead of or in addition to the conductive objects 201, insulating objects 203 may be used as shown in the figure and in such a case, the insulating objects 203 are coated with a conductive thin layer 202 for practical use. A layer of the bonding agent 101 has a thickness of about 0.5 .mu.m, and the conductive objects 201 and the coating of the insulating object 203 with the conductive thin layer 202 have a length (or maximum dimension) of about 1.0 .mu.m so that a sufficient quantity of the conductive objects 201 are exposed. An actual field emitting device is assembled by adding an anode and a gate to the emitter section.
The resulting field emitting device is shown in FIG. 40 wherein an insulating layer 409 is formed on the support 100 having a plurality of emitter objects 201 borne thereon, with some of the emitter objects 201 left uncovered. On the insulating layer 409 is formed a conductive layer 401 functioning as a gate for adjusting electron flow. On the conductive layer 401 is formed an insulating layer 402. Disposed on the insulating layer 402 is a screen 404 having an anode function too. A luminescent layer 403 is formed on that side of the screen 404 which faces the emitter objects 201. The screen 404 is coupled in vacuum as by soldering, with the encapsulated areas 406 being evacuated. Voltage application causes the emitter objects 201 to emit electrons and by the action of emitted electrons, light emission 408 occurs through the screen 404.
In the device disclosed in this patent, since there are some sites where the emitter objects 201 are in contact with the insulating layer 409 as is evident from FIG. 40, on voltage application, the voltage can concentrate at the insulating layer 409, increasing the risk of breakage. If the insulating layer 409 is made thick in order to prevent such breakage, the voltage applied for electron emission must be undesirably increased.